Sound recording



Jan, 14, 1930. c. R. KEIJ'H SOUND RECORDING Filed May 4, 1928 III Fla. 5

6 5 4 utbwhhtm utmbouw w I! FREaL/nv'a Y /N K. a.

FREQUENCY IN lT-G.

llu

Fla. 4

FREQUENCY /N KG.

FREQUENCY m/ ntc.

m w a n m m u .uqbb mtw INVENTUR CLYDE fi. fl'E/TH Patented Jan. 14, 1930 UNITED STATES PATENT OFFlCE CLYDE R. KEITH, OF EAST ORANGE, NEW JERSEY, ASSIGNOR TO BELL TELEPHONE" LABORATORIES, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK souzun ancoanme I Application filed May 4, 1928. Serial No. 275,083.

This invention relates to sound recording and reproducing systems and more particularly to such systems wherein, sound currents are modulated before recording. 7

It is well known in the art of sound reproduction that for constant sound intensity over the range of speech and music the air pressure at the diaphragm,and hence the energy level, must be increased very rapidly as the frequency falls below 1000 cycles. Fromthis it follows that the record groove for low frequencies must be many times the amplitude required for high frequencies. amplitude ratio introduces a serious limitation in recording for when a wide band of frequencies is to be covered the energy level is'limited by the tendency of the stylus at low frequencies to cut over into an adjacent groove.

It is the object of this invention to overcome this objection and provide a method of recording a wide range of'frequencies at a high energy level ation in the record trace amplitude.

In accordance with the general features of the invention the currents representing the sounds to be recorded are inverted on the frequency scale so that the componentsof high energy are given a high frequency and the normally low energy components are transposed to the low frequency band, As a result of this inversion the high stylus velocity required at low frequencies is obtained with much lower displacement and the variation between maximum and minimum displacement is much reduced.

This invention may be more readily understood from the accompanying drawing and the following description of one manner in which it may be carried out. Fig. 1 of the drawing shows diagrammatically a system for recording a sideband of a carrier modulated by currents corresponding to sound waves and Fig. 2 indicates a system for re producing recordsmade by the system of Fig. 1. Figs. 3 and 4 show the relation between acoustic pressure and frequency and the necessary record trace amplitude and frequency for recording the various frequencies of speech and music by ordinary This high with only a moderate vari-' no detailed description of their operation is -frequency as I of the frequencies recording methods. Figs. 5 and 6 apply to the corresponding sideband offrequencies recorded the present invention and show the large reduction in record trace amplitude when the frequency order is inverted.

The recordingand reproducing systems used for illustrating this invention are well known to those skilled in the art and a'detailed description of them may be found in Mills Patent 1,603,287, granted October,19, 1926. In the'present case therefore only a diagrammatic showing ofthese systems is made. In Fig. 1, 1, is a sound pick-up device; 2, represents the modulating system including all necessary filters, transformers, etc; 3, thesource of carrier frequency; and 4, the electrical recorder. Likewise in the re producing system of Fig. 2, 5, is the reproducer for retranslating into electrical currents the waveform recorded on record-6; 7, the amplifying and modulating systems; 8, the source of carrier frequency; and 9, a suitable receiver for reproducing the original sounds. f The operation of these systems is identical with the description given'in the Mills patent previously mentioned and since the particular modulating and demodulating systemsemployed form no part of'the present invention,

given. If thecarrier frequency is made equal to the highest frequency in the wave form to be recorded the lower sideband delivered to the recorder will be composed of the same the original band but the order will be reversed or inverted. Since it is proposedaccording to this invention to record a wide range of frequencies, it would be an advantage to employ a carrier of somewhat higher frequency than the upper limit of the band to be recorded. As stated in the application of J. W. Horton, Ser1al N 'o. 27 5,409, filed May :5, 1928, this resultsin dlslacing as well as inverting the band on the requency scale in such a way that, the ratio between the upper and lower limits is reduced so that the recording and reproduclng mechanisms are more uniformly responsive to the frequencies involved.

If in the present case, for instance, fre- I quencies from 100 to 3000 cycles are to be recorded, this ratio is 30 when the wave form is merely inverted, but when a-F3500 c cle carrier is used the lower side band wi 1 ran e from 3400 to 500 cycles which means that t 1e ratio is reduced to approximately 7.

For the purposes of illustrating the resent-invention, however, the curves of igs.

3 to 6 in the drawings have been shown for inversion only in order that direct comparison between the inverted and uninverted high energy com onents may be obtained at transposed in the manner described.

s The very marked reduction in maximum amplitude and the more nearly constant aver age value obtained are shown by the curves of FigsA an'd6 which are plotted in the same units. A further ain in this respect may be obtained by slight y attenuating the currents representing thesounds of highest fre uencysuch as between 2800 and 3000 cycles 1n the present instance, so that the amplitude of the record trace at the lowest frequenc recorded does not exceed that required for t e highest frequency. This result is indicated by the dotted portion of the curve in Fi lower trace amplitude if the components are 6. This arbitrary loss inthe recor ling circuit is by no means as objectionable as a similar loss would be in normal recording for it should be observed that due'to the frequency inversionthe sound frequencies affected are those between 2800 and 3000 rather than those Accordingly, I

at the lower end of the scale. the low frequencies, which are entirely'lost or greatly attenuated with ordinary recording methods, are reproduced at natural intensity and the loss in quality due to the attenuation is negligible. 7 3

As a result therefore of the inversion and reinversion of the frequenciesinthe speech or music,-it is possible to use a eat dealmore energy in recording on stem ard records or to increase the playing time withoutsacrificing quality by reducing the spacin'gof the record grooves. 1

What is claimed is:

1. A method of recording wave' formsiembracing a wide range of frequencies at substantiall constant amplitude of the record trace w ich consists in shifting the wave form upward on the frequency-scale and recording them in inverted frequency order.

2; A method of increasing the effective cashifting upward on the frequency scale and inverting the frequency order of the wave forms to be recorded whereby the low frequency components of said forms may be represented by a record trace substantially equal in amplitude to that required for the high frequency components.

3. A method whereb wave forms may be recorded at substantia ly the maximum capacity of the recording medium for all frequencies in said forms which consists in displacing the frequencies and inverting the frequency order of said forms before recording.

and adjusting the energy level in the recording circuit in accordance with the maximum capacity of said medium.

In Witness whereof, I hereunto subscribe my name this 2 day of May, 1928.

CLYDE R. KEITH. 

